The question of how much a lipid monolayer at the air-water interfece should be compressed to yield a molecular packing arrangement corresponding to that of the same lipid in a bilayer has remained unanswered since Gorter and Grendel raised it 60 years ago in their paper proposing the bilayer as the structural basis of biological membranes. Estimates of this pressure, here termed the isomorphic pressure, range from below 30 to 50 dynes/cm, a differnce in bilayer surface energy of over 4 kcal/mole. The answer to the question if important because monolayers can be used as models for bilayers and hence, natural membranes. When properly examined, they can provide information difficult to obtain otherwise, in particular, physical properties that are important in the energetics of the pentration of proteins and hydrophobic molecules as well as a fundamental property of bilayers, the energy of the aqueous interface. In addition, monolayers are the starting point for many membrane reconstitution methods, the success of which will benefit from greater understanding of the relationship between bilayers and monolayers. The goal of tis proposal are (I) to establish the isomorphic pressure for common lipids and their mixtures, (II) to explore some of the properties of such, and (III) make use of monolayer properties to understand come bilayer characteristics that have important physiological relevance. The approach to Goal I will be to vary the surface pressure until monolayer and bilayer correspond with respect to phase transition temperature, 2-dimensional diffusion coefficient and partition coefficient of a hydrophobic probe. By comparing this pressure with the equilibrium surface pressure of bilayer vesicles, the energy of interaction across the bilayer will be obtained. This energy is expected to be small, in which case, measurement of the equilibrium pressure of a membrane (a very simple procedure) will provide a good estimate of the isomorphic pressure. To meet Goal II, the basic physical properties (area per molecule, compressibility, surface potential and macroviscosity) or isomorphic monolayers will be established using standard surface chemical techniques. The approach to Goal III will be to test a prediction about fundamental properties of bilayers, namely that the extent of membrane penetration by a foreign molecule (here, fatty acids and a toxin) increases as the equilibrium surface pressure of the penetration approaches the surface pressure of the membrane (measured as the isomorphic monolayer pressure) and that the rate of penetration increases in addition with increasing compressibility of the membrane. The large difference between the equilibrium surface pressures of cis- and trans-unsaturated fatty acids and the large temperature coefficient of the latter allow convenient testing of the influence of penetrant surface pressure. Melittin will be used to assess dependence of penetration on surface pressure of the target membrane. Phospholipid transfer will be measured using small vesicles, which because their surface pressure decreases with their radius, provide a convenient system to investigate self-penetration, where 2-dimensional mixing is necessarily ideal. Finally, one aspect of environmental effects on surface pressure, that of osmotic pressure, will be examined.